1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell stack, a combustor, a heat exchanger, and heat utilization equipment. Further, the present invention relates to a method of operating the fuel cell system.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive solid oxide such as stabilized zirconia. The solid electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
Since the fuel cell stack of this type is operated at high temperature (e.g., about 800° C.), the fuel cell is suitably adopted in cogeneration, e.g., for heating water using the waste heat to supply hot water.
For example, in a fuel cell cogeneration system disclosed in Japanese Laid-Open Patent Publication No. 2001-185167, as shown in FIG. 11, an off gas burner 2a for burning the off gas from a fuel cell 1a is provided downstream of the fuel cell 1a. A tank 3a filled with hot water is provided in a water heating system using the waste heat of the off gas burner 2a for heating a heat medium. An additional heating burner 5a is provided in a water circuit 4a connected to the tank 3a. Oxidation catalyst for burning a combustion fuel (city gas) is filled in the additional heating burner 5a. 
According to the disclosure, it is possible to heat water in the tank 3a using the waste heat of the off gas burner 2a and the combustion heat of the additional heating burner 5a, and the high temperature of the water in the tank 3a is maintained at all times.
Further, Japanese Laid-Open Patent Publication No. 2006-073316 discloses a fuel cell cogeneration system and a method of controlling the fuel cell cogeneration system. As shown in FIG. 12, the fuel cell cogeneration system includes a fuel cell 1b operated at high temperature, a heat exchanger 3b provided in an exhaust gas system 2b of the fuel cell 1b for supplying the waste heat from the fuel cell 1b to heat demanding components, monitor means 4b for monitoring the operating state of the fuel cell 1b, and a control unit 5b. The control unit 5b implements control for changing the power generation output of the fuel cell 1b and/or the exhaust gas temperature of the fuel cell 1b in correspondence with the change in the heat demand.
However, in Japanese Laid-Open Patent Publication No. 2001-185167, two burners, i.e., the off gas burner 2a and the additional heating burner 5a need to be provided. Therefore, a large space is required for these burners, and it is difficult to reduce the overall size of the system, and the cost of the system is high.
Further, in Japanese Laid-Open Patent Publication No. 2006-073316, since the fuel utilization ratio or the like is changed depending on the heat demand, the amount of the fuel gas supplied to the fuel cell 1b is changed. Thus, the conditions in power generation reactions in the fuel cell 1b (reaction temperature and current density) are not stable.